Development of a Pre-screening Method for Storage of Grafted Cucurbitaceae Seedlings

Grafting is a unique horticultural technology that enhances plant growth and development, and facilitates sustainable crop production practices by reducing rates and overall use of soil fumigants as an alternative to manage soil borne diseases. However, vegetable grafting demands an intensive propagation process. Short-term storage of grafted seedlings up to several weeks was successfully demonstrated as a possible solution to limited production capacity in United States relative to potentially large demand (e.g., Justus and Kubota, 2010). However, optimum storage temperature is specific for each species and cultivar and a standardized temperature pre-screening method using scion and rootstock genotypes can be helpful to develop an efficient short-term storage technique for grafted seedlings. In this study, we examined seedling radicle elongation rates at different temperatures for evaluating genotype-specific chilling sensitivity. Seeds of ‘Tetsukabuto’ squash rootstock (Cucurbita maxima x Cucurbita moschata), ‘Olympic Gold’ muskmelon scion (Cucumis melo) and ‘Sweet Harmony’ watermelon scion (Citrullus lanatus) were germinated at 25°C under darkness inside a pair of vertically oriented Plexiglas sheets holding seeds against moist paper towel. After the radicle reached 5-10 mm long, the seedlings were subjected to one of three different temperatures (5°C, 12°C, or 25°C) in darkness. After 72 hours, all seedlings were transferred to a recovery treatment at 25°C until their roots reached 100 mm long.  Root growth of ‘Tetsukabuto’, ‘Olympic Gold’ and ‘Sweet Harmony’ seedlings were significantly lower at 5°C and 12°C than that at 25°C. ‘Tetsukabuto’ seedlings had 2.47 times higher rate of radicle growth than ‘Olympic gold’ at 5°C, and 6.10 and 6.17 times higher rate of growth than ‘Sweet harmony’ at 5°C and 12°C, respectively. At 12°C,  no significant difference were found between ‘Testsukabuto’ and ‘Olympic gold’ showing ‘Sweet harmony’ as the most chilling sensitive. Under the recovery following the chilling treatment, there was no significant difference between the genotypes; however, ‘Tetsukabuto’ seedlings radicle length reached 100 mm faster than the others. The results obtained in this experiment demonstrated that ‘Testukabuto’ was highly tolerant and ‘Sweet harmony’ was most sensitive to low temperatures, which is in general agreement with the results of our previous research comparing the storability of the same three genotypes at low temperatures (Spalholz, 2013). In conclusion, root growth rate of young seedlings can be used as a temperature pre-screening method for testing storability of cucurbitaceae genotypes used for scion and rootstock.